Objective
We sought to examine the relation between recommended levels of physical activity during pregnancy and pregnancy outcomes.
Study Design
We conducted an observational study with energy expenditure, aerobic fitness, and sleeping heart rate measured in 44 healthy women in late pregnancy. Medical records were examined for pregnancy outcome.
Results
Active women, who engaged in ≥30 minutes of moderate physical activity per day, had significantly better fitness and lower sleeping heart rate compared to the inactive. Duration of second stage of labor was 88 and 146 minutes in the active vs inactive women, respectively ( P = .05). Crude odds ratio of operative delivery in the inactive vs the active was 3.7 (95% confidence interval, 0.87–16.08). Birthweight, maternal weight gain, and parity adjusted odds ratio was 7.6 (95% confidence interval, 1.23–45.8). Neonatal condition and other obstetric outcomes were similar between groups.
Conclusion
Active women have better aerobic fitness as compared to inactive women. The risk for operative delivery is lower in active women compared to inactive, when controlled for birthweight, maternal weight gain, and parity. Further studies with larger sample size are required to confirm the association between physical activity and pregnancy outcomes.
Traditionally, pregnant women are advised to reduce physical activity levels. This advice was based on concerns that exercise would negatively affect pregnancy outcomes by raising core body temperature, by increasing the risk of maternal musculoskeletal injury due to changes in posture and ligamentous laxity, and by shunting transport of oxygen and nutrients to maternal skeletal muscle rather than to the developing fetus.
Research has provided new information on how pregnant women and their fetuses respond to moderate physical activity, showing no adverse maternal or neonatal outcomes. Moreover, regular moderate physical activity has been shown to result in marked benefits for mother and fetus. Maternal benefits include improved cardiovascular function, limited pregnancy weight gain, decreased musculoskeletal discomfort, reduced incidence of muscle cramps and lower limb edema, mood stability, a reduction of gestational diabetes and gestational hypertension, and less complicated labor. Fetal benefits include decreased fat mass, improved stress tolerance, and advanced neurobehavioral maturation. Based on these findings, the American College of Obstetricians and Gynecologists (ACOG) recommends that pregnant women who are free of medical or obstetric complications follow the American College of Sports Medicine–Centers for Disease Control and Prevention general guidelines for physical activity, and engage in ≥30 minutes of moderate physical activity per day on most, if not all, days of the week.
However, these recommendations are not universally accepted. Some investigators argue that the ACOG guidelines may be too cautious and that there is no firm evidence that exercise at higher intensities produces any harmful effects on the fetus. On the other hand, some authors are reluctant in recommending physical activity during pregnancy, because of the insufficient evidence about benefits and risks.
More information is needed to evaluate the influence of the recommended levels of physical activity on pregnancy, labor, and delivery. So far, only a limited number of studies were conducted on this topic, involving small groups of athletes whose exercise habits do not correspond to physical activity levels recommended for the general population. Moreover, it is difficult to accurately assess free-living physical activity, both in general and during pregnancy. This contributes to the lack of valid evidence on which recommendations could be based.
We sought to test the hypothesis that engaging in the recommended ≥30 minutes of moderate physical activity per day in late pregnancy is not associated with detrimental effects for pregnant women or their infants. Furthermore, we hypothesized that such physical activity levels provide substantial benefits across a broad range of outcomes, including higher cardiovascular fitness and improved delivery outcome.
Materials and Methods
Study protocol
During routine medical examinations at the maternity unit of the University Hospitals of Geneva, Switzerland, participation in this observational study was proposed to healthy women in the third trimester of pregnancy. A total of 71 women consented to participate and were enrolled in the study at baseline, which was approximately 25% of the women solicited. The study was approved by the institutional ethics committee, and all participants gave written informed consent.
The majority of pregnant women delivering in our hospital are followed up by their private gynecologists. In case of uncomplicated pregnancy, they consult the hospital clinic during the last trimester of pregnancy to prepare for delivery. Women were informed about our study at their first visit to the hospital. Consenting women were scheduled to assess resting metabolic rate, free-living total energy expenditure, activity-related energy expenditure, maximal oxygen uptake (V̇O 2 max), sleeping heart rate, and movement (accelerometry). Criteria for exclusion from the study were: heart disease or treatment that may alter cardiovascular conditioning, preeclampsia, diabetes, risk for premature delivery, high probability of cesarean section (breech or transverse lie, previous cesarean), and fetal malformation or growth retardation. All participating subjects were in good health and took no medications.
Out of 71 women enrolled in the study, 18 delivered before prelabor measurements could be completed. Incomplete measurements were due to delivery before completing the 3-day heart rate recordings or before the date of the appointment for calorimetry. Data from 8 women were excluded due to incomplete 3-day heart rate recordings because of technical reasons, and 1 due to incomplete medical record. Data for the remaining 44 subjects were used for statistical analysis.
Anthropometric measurements
Body weight was measured to the nearest 0.1 kg on a calibrated beam scale (Seca, Hamburg, Germany) and body height to the nearest 0.5 cm with a height rod (Seca) with the subjects in light clothing and without shoes. Total weight gain was computed as the difference in weight at delivery minus prepregnancy weight. Gestational age was assessed based on the last menstrual period, or based on a first-trimester ultrasound measurement if a difference of >1 week between the 2 estimates was detected. Birthweight was measured by an experienced midwife upon delivery.
Indirect calorimetry measurements
Resting metabolic rate was assessed by indirect calorimetry using a ventilated hood system (Deltatrac II metabolic monitor; Datex-Ohmeda, Helsinki, Finland). The gas analyzer was calibrated before each measurement according to the manufacturer’s instructions. Women arrived at the hospital in the morning, after an overnight fast, avoiding any strenuous physical effort. After relaxing on a bed for 30 minutes, the hood was placed over their heads and measurements were started. Oxygen consumption (V̇O 2 ) and carbon dioxide (V̇CO 2 ) production were measured for 30 minutes and averaged at 1-minute intervals with the subjects in a supine position and completely at rest in a thermoneutral environment (20-22°C). The first 5 minutes of data were eliminated as acclimatization artefact. From the remaining 25 minutes a segment of 5 consecutive 1-minute measures with <10% coefficient of variation in V̇O 2 and V̇CO 2 was considered as steady state. V̇O 2 and V̇CO 2 were then used to calculate resting metabolic rate using the abbreviated Weir equation. This method was shown to be a valid method for resting energy expenditure measurements. The mean errors for V̇O 2 and V̇CO 2 estimates were shown to be 1.9% and 1.5%, respectively.
Total and activity-related energy expenditure
Total energy expenditure and activity-related energy expenditure were estimated analyzing 24-hour recordings of heart rate and movement. A lightweight (10 g) waterproof combined heart rate and movement sensor (Actiheart; Cambridge Neurotechnology Ltd, Papworth, United Kingdom) was clipped onto 2 standard electrocardiography electrodes and worn on the chest day and night.
The reliability and validity of the device have been published elsewhere. The Actiheart was shown to give accurate estimates against indirect calorimetry during a wide range of activities in men and women (from low, through moderate and high activities) in both laboratory and field settings.
The mean errors of the individually calibrated estimates were shown to be 1.5% in nonpregnant subjects. To account for possible pregnancy-induced changes, we performed an individual calibration using a step test (see “Cardiovascular fitness measurements”).
This device simultaneously measured body acceleration and heart rate in the women during 5 consecutive days with a 30-second data epoch setting. During these 5 days the women were asked to continue their usual life routine and physical activity habits. From these measurements, 3 full 24-hour recordings were obtained and used for the energy expenditure estimation.
Activity-related energy expenditure was calculated by subtracting resting metabolic rate (estimated using indirect calorimetry, as described above) and dietary-induced thermogenesis (estimated as 10% of total energy expenditure) from total energy expenditure. Total intensity of physical activity measured by the Actiheart was then transformed in metabolic equivalents (METs) calculated as a multiple of resting metabolic rate. Physical activity level was calculated as the ratio of total energy expenditure (sum of resting metabolic rate, diet-induced thermogenesis, and activity energy expenditure) over resting metabolic rate. Movement was expressed as the mean count of vertical accelerations per minute detected by the accelerometer during 3 full days.
Moderate physical activity was defined as any activity between 3-6 METs (or multiples of resting metabolic rate). Based on the ACOG recommendations, the women were classified into 2 groups: active (≥30 minutes of moderate physical activity per day) and inactive (<30 minutes of moderate physical activity per day).
Cardiovascular fitness measurements
Fitness level was estimated with a step test. Women stepped up and down a 15-cm step progressively increasing step frequency from 15-32.5 body lifts per minute (rate of change: 2.5 body lifts/min ). They were advised to stop the test if they felt uncomfortable. The mass-specific lift work rate (mechanical power) of the step test was calculated as 9.81 m/s 2 × step height (m) × lift frequency (number of body weight lifts/min) and expressed in J/min/kg. Linear regression was used to model mass-specific lift work rate from heart rate during stepping. The resulting regression line was then extrapolated to the assumed maximum heart rate for the person’s age using the equation of Tanaka et al, from which V̇O 2 max was estimated.
Sleeping heart rate was defined as the highest value among the 60 lowest heart rate recordings during a 24-hour period. Mean sleeping heart rate was determined as the average value of the 3 daily sleeping heart rate recordings.
Clinical outcomes
Information on the duration of labor (first and second stages), duration of pushing efforts, episiotomy or perineal laceration, 5-minute Apgar scores, presence of meconium, type of anesthesia, postpartum hemorrhage, and mode of delivery (spontaneous, vacuum, forceps, cesarean section) was collected from the medical records. First stage of labor was defined as the duration from diagnosis of labor (regular contractions, progressive dilatation of the cervix) to full dilatation of the cervix. Second stage of labor was from full dilatation to delivery. Duration of pushing efforts was from starting active pushing to delivery.
Statistical analysis
Differences between groups were tested using unpaired Student t test and Fisher’s exact test. Mean duration of the first and second stage and of pushing efforts were evaluated with time-to-event curves (Kaplan-Meier) and compared using the log rank test. Incomplete durations because of cesarean section or operative delivery were treated as censored observations. Logistic regression analysis was used to examine the independent association of physical activity with delivery mode. Crude and adjusted odds ratios (OR) and their 95% confidence intervals (CIs) were computed. Software (SPSS, version 15; SPSS Inc, Chicago, IL) was used for all statistical analysis.
The initial sample size was approximately 20 active and 20 inactive women. Level of activity was based on women’s self-reporting of activity level. This sample size was sufficient to show a difference in continuous measurements of 1 SD between groups, with a power of 90% and 95% confidence level. Given the high attrition, we continued the enrollment to a total of 71 women, to be able to analyze complete data from the number of women initially planned.
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